This invention relates to cellular communication systems, and more particularly to an antenna apparatus for use by mobile subscriber units to provide beamforming transmission and reception capabilities.
Code Division Multiple Access (CDMA) communication systems may be used to provide wireless communication between a base station and one or more mobile subscriber units. The base station is typically a computer controlled set of transceivers that are interconnected to a land-based public switched telephone network (PSTN). The base station includes an antenna apparatus for sending forward link radio frequency signals to the mobile subscriber units. The base station antenna is also responsible for receiving reverse link radio frequency signals transmitted from each mobile unit. Each mobile subscriber unit also contains an antenna apparatus for the reception of the forward link signals and for transmission of the reverse links signals. A typical mobile subscriber unit is a digital cellular telephone handset or a personal computer coupled to a cellular modem. In CDMA cellular systems, multiple mobile subscriber units may transmit and receive signals on the same frequency but with different codes, to permit detection of signals on a per unit basis.
The most common type of antenna used to transmit and receive signals at a mobile subscriber unit is a mono- or omni-pole antenna. This type of antenna consists of a single wire or antenna element that is coupled to a transceiver within the subscriber unit. The transceiver receives reverse link signals to be transmitted from circuitry within the subscriber unit and modulates the signals onto the antenna element at a specific frequency assigned to that subscriber unit. Forward link signals received by the antenna element at a specific frequency are demodulated by the transceiver and supplied to processing circuitry within the subscriber unit.
The signal transmitted from a monopole antenna is omnidirectional in nature. That is, the signal is sent with the same signal strength in all directions in a generally horizontal plane. Reception of a signal with a monopole antenna element is likewise omnidirectional. A monopole antenna does not differentiate in its ability to detect a signal in one direction versus detection of the same or a different signal coming from another direction.
A second type of antenna which may be used by mobile subscriber units is described in U.S. Pat. No. 5,617,102. The system described therein provides a directional antenna comprising two antenna elements mounted on the outer case of a laptop computer. The system includes a phase shifter attached to the two elements. The phase shifter may be switched on or off in order to affect the phase of signals transmitted or received during communications to and from the computer. By switching the phase shifter on, the antenna transmit pattern may be adapted to a predetermined hemispherical pattern which provides transmit beam pattern areas having a concentrated signal strength or gain. The dual element antenna directs the signal into predetermined quadrants or hemispheres to allow for large changes in orientation relative to the base station while minimizing signal loss.
CDMA cellular systems are also recognized as being interference limited systems. That is, as more mobile subscriber units become active in a cell and in adjacent cells, frequency interference becomes greater and thus error rates increase. As error rates increase, maximum data rates decrease. Thus, another method by which data rate can be increased in a CDMA system is to decrease the number of active mobile subscriber units, thus clearing the airwaves of potential interference. For instance, to increase a current maximum available data rate by a factor of two, the number of active mobile subscriber units can be decreased by one half. However, this is rarely an effective mechanism to increase data rates due to a lack of priority amongst users.
Problems of the Prior Art
Various problems are inherent in prior art antennas used on mobile subscriber units in wireless communications systems. One such problem is called multipath fading. In multipath fading, a radio frequency signal transmitted from a sender (either base station or mobile subscriber unit) may encounter interference on route to an intended receiver. The signal may, for example, be reflected from objects such as buildings that are not in the direct path of transmission, but that redirect a reflected version of the original signal to the receiver. In such instances, the receiver receives two versions of the same radio signal; the original version and a reflected version. Since each received signal is at the same frequency but the reflected signal may be out of phase with the original due to reflection and a longer transmission path, the original and reflected signals may tend to cancel each other out. This results in fading or dropouts in the received signal, hence the term multipath fading.
Single element antennas are highly susceptible to multipath fading. A single element antenna has no way of determining the direction from which a transmitted signal is sent and cannot be tuned or attenuated to more accurately detect and receive a signal in any particular direction.
The dual element antenna described in the aforementioned reference is also susceptible to multipath fading, due to the symmetrical nature of the hemispherical lobes formed by the antenna pattern when the phase shifter is activated. Since the lobes created in the antenna pattern are more or less symmetrical and opposite from one another, a signal reflected in a reverse direction from its origin can be received with as much power as the original signal that is directly received. That is, if the original signal reflects from an object beyond or behind the intended receiver (with respect to the sender) and reflects back at the intended receiver from the opposite direction as the directly received signal, a phase difference in the two signals can create a multipath fading situation.
Another problem present in cellular communication systems is intercell interference. Most cellular systems are divided into individual cells, with each cell having a base station located at its center. The placement of each base station is arranged such that neighboring base stations are located at approximately sixty degree intervals from each other. In essence, each cell may be viewed as a six sided polygon with a base station at the center. The edges of each cell adjoin each other and many cells form a honeycomb like image if each cell edge were to be drawn as a line and all cells were viewed from above. The distance from the edge of a cell to its base station is typically driven by the maximum amount of power that is to be required to transmit an acceptable signal from a mobile subscriber unit located near the edge of a cell to that cell""s base station (i.e., the power required to transmit an acceptable signal a distance equal to the radius of one cell).
Intercell interference occurs when a mobile subscriber unit near the edge of one cell transmits a signal that crosses over the edge of a neighboring cell and interferes with communications taking place within the neighboring cell. Typically, intercell interference occurs when similar frequencies are used for communication in neighboring cells. The problem of intercell interference is compounded by the fact that subscriber units near the edges of a cell typically use higher transmit powers so that the signals they transmit can be effectively received by the intended base station located at the cell center. Consider that another mobile subscriber unit located beyond or behind the intended receiver may be presented at the same power level, representing additional interference.
The intercell interference problem is exacerbated in CDMA systems, since the subscriber units in adjacent cells may typically be transmitting on the same frequency. What is needed is a way to reduce the subscriber unit antenna""s apparent field of view, which can have a marked effect on the operation of the forward link (base to subscriber) by reducing the apparent number of interfering transmissions. A similar improvement is needed for the reverse link, so that the transmitted signal power needed to achieve a particular receive signal quality could be reduced.
The present invention provides an inexpensive antenna apparatus for use with a mobile subscriber unit in a wireless same frequency communication system, such as a CDMA cellular communication system.
The invention provides a precise mechanism for determining in which direction the base station assigned to that unit is located and provides a means for configuring the antenna apparatus to maximize the effective radiated and/or received energy. The antenna apparatus includes at least one active antenna element that transmits and receives RF energy, multiple passive antenna elements that re-radiate the RF energy, and a like number of selective impedance components, each respectively coupled to one of the passive antenna elements. The selectable impedance components are independently adjustable (i.e., programmable) to affect the direction of the beam produced by the directive antenna. Thus, forward and reverse links have improved gain.
The selectable impedance components are independently adjustable to make the associated antenna elements reflective or transmissive. Reflective antenna elements are, in effect, elongated, causing reflection of RF signals. Transmissive antenna elements are, in effect, shortened, allowing RF signals from the active antenna element(s) to propagate past them. Through proper coordination of the passive antenna elements, the subscriber unit uses the directive antenna to direct the beam to reduce multipath fading and intercell interference.
In one embodiment, the antenna apparatus is allowed to adapt to various orientations with respect to the base station. In this embodiment, the antenna apparatus also includes a controller coupled to the selectable impedance components. The controller determines an optimal impedance setting for each selectable impedance component. The proper phase, set by the associated impedance component, of each passive antenna element may, for example, be determined by monitoring an optimum response to a pilot signal transmitted from the base station. The antenna apparatus thus acts as a beamformer for transmission of signals from the subscriber unit and acts as a directive antenna for signals received by the subscriber unit.
Through the use of an array having at least one active antenna element and multiple passive antenna elements each having a programmable re-radiation phase, the antenna apparatus is estimated to increase the effective transmit power per bit transmitted by as much as 3 decibels (dB) for reverse link communications over classic phased array antenna configurations, which provide 4.5 dBi. Thus, the number of active subscriber units in a cell may remain the same while the antenna apparatus of this invention increases data rates for each subscriber unit beyond those achievable by prior art antennas. Alternatively, if data rates are maintained at a given rate, more subscriber units may become active in a single cell using the antenna apparatus described herein. In either case, the capacity of a cell is increased, as measured by the sum total of data being communicated at any moment in time.
Forward link communication capacity can be increased as well, due to the directional reception capabilities of the antenna apparatus. Since the antenna apparatus is less susceptible to interference from adjacent cells, the forward link capacity can be increased by adding more users or by increasing cell radius size.
With respect to the physical implementation of the antenna apparatus, one embodiment of the invention specifies that a central, active, antenna element is encircled by multiple passive antenna elements mounted on a planar surface having a single ground plane layer. Electrical coupling to the ground plane is implemented through switches coupling the associated antenna elements to respective, fixed, impedance components, such as a delay line, capacitor, inductor, or adjustable impedance component, such as a varactor. Other embodiments specify that more than one active antenna element is employed along with an associated feed network, forming an antenna array surrounded by multiple, passive, antenna elements.